Derivatives of 4-fluoro-2-hydroxymethylene-3-oxobutyrates, in particular the enol ethers of formula III above, wherein R4 is lower alkyl, are valuable intermediates in the synthesis of heterocyclic compounds such as pyrazoles (see e.g. JP 01-113371 A, U.S. Pat. No. 5,093,347, WO 2005/123690 A1). A known synthesis (cf. WO 2005/123690 A1) of said enol ethers is based on the reaction of the corresponding 3-oxobutyrates with trialkyl orthoformates (HC(OR)3), which are relatively expensive, in the presence of acetic anhydride. The orthoformate and acetic anhydride are both used in excess. Moreover, the process suffers from poor atom economy because only one of the three alkoxy groups of the trialkyl orthoformate remains in the product and the other two combine with acetic anhydride to give acetic acid and the corresponding alkyl acetate as byproducts.
Accordingly, it was an object of the present invention to provide an alternative method for the production of the enolates and/or enol ethers or esters of formulae I and III above, which has an improved atom economy and does not require expensive reagents.